Head-mounted image display devices that are worn on the head and used to view images, or in other words, head-mounted displays, are known. A head-mounted image display device has an image display unit for each of the left and right eyes, in general, and by combining these with headphones, the head-mounted image display device is configured to enable control of visual perception and auditory perception. In addition, a head-mounted image display device is also capable of showing different images to the left and right eyes, and if an image with parallax between the left and right eyes is displayed, a three-dimensional image may be presented.
For the display unit for the left and right eyes, the head-mounted image display device is equipped with a high-resolution display panel made of liquid crystals or organic electroluminescence (EL) elements, for example. Head-mounted image display devices may be categorized into opaque and transmissive types. An opaque head-mounted display device is configured to directly cover the user's eyes when mounted on the head, and increases immersion during image viewing. A suitable angle of view is set with an optical lens that projects a display screen. In addition, if multichannel audio is reproduced with headphones, it is possible to recreate the sense of being in a movie theater (for example, see Patent Literature 1). On the other hand, in the case of a head-mounted image display device of the transmissive type, it is possible to look at the outside surroundings past the image (that is, see through the image) even while the user is wearing the device on his or her head and an image is being displayed (for example, see Patent Literature 2). For this reason, the user is able to avoid dangers such as collisions with obstacles while using the device outdoors or while walking.
Meanwhile, with regard to flat-panel display devices, there is proposed a display device capable of displaying an image on both the front and back sides (for example, see Patent Literature 3 and 4). A double-sided display device may display the same content on both sides, or display separate content on both sides. Since the user may view an image both the front side and the back side of the device housing, a double-sided display device can be a good information providing tool.
On the other hand, the head-mounted image display device discussed above may display an image on the front side, that is, facing the eyes of the wearer, but does not display an image on the back side, that is, outward from the image display device. For this reason, people on the outside are completely unable to check what the wearer is seeing. Also, since the wearer's eyes are covered, it is difficult for people on the outside to grasp the status of the wearer (for example, whether it is alright to speak now or wait until later).
It can be argued that a head-mounted image display device is an ultimate personal display. However, people other than user him- or herself who is wearing the device are unable to know the status of the user, what the user is doing, what the user is viewing, or the degree to which the user is concentrating on or immersed in viewing. Furthermore, in some cases the user him- or herself may want to announce his or her own status, inform others of the content being viewed, or communicate useful information related to the content, but to do this the user must first remove the device, and viewing is interrupted.